2000 Dodge Caravan P1595

My Caravan Has served me well and still has the original Motor and Transmission and till now hasn't given me very much trouble other than regular mantiance. Today I stared up a hill and could not go more than 20 mph. The transmission kicked down and speed stayed low. At the top of the hill all was fine again. The van went completely beserk speed odometer jumping all gauges active idiot lights flashing radio on and off on it's own wipers and turn signals too ! Then after a few seconds back to normal I does seem to lack som e power but not much. No more crazy lights or anything Cat has been replaced and I have code p1595 Help what does this mean and where do we start.